1. Field of the Invention
This invention relates to a method for treating a subterranean reservoir penetrated by a well wherein, as part of the treatment, steam is injected into the reservoir. More particularly, the invention relates to such a method involving inhibiting the dissolution of silica from the permeable reservoir rock or from a gravel pack positioned in the well opposite the portion of the reservoir into which steam is injected.
2. Description of the Art
Various treatments of permeable subterranean reservoirs penetrated by one or more wells involve injection of steam into the reservoir. Typical reservoirs include hydrocarbon-producing reservoirs, particularly those reservoirs containing heavy oil, bitumen, tar or similar viscous hydrocarbons. An example of one type of treatment involves the injection of steam into a portion of a reservoir to raise the temperature and consequently decrease the viscosity of the viscous hydrocarbons contained in the reservoir. Subsequently, these hydrocarbons can be more easily moved through the reservoir to a well through which they can be produced to the surface of the earth. Such treatments can be of the type referred to as a steam drive wherein steam is injected via one well and hydrocarbons are pushed through the reservoir and produced via another well. Alternatively, the treatments can be of the cyclic steam injection type involving a single well wherein steam is injected for a first period of time following which the same well is produced for a second period of time.
A steam source provides steam for injection into the reservoir. The steam source is most often a fuel-fired steam generator capable of producing either essentially dry steam, containing as much as 95 percent by weight of vapor, or wet steam such as steam containing various amounts of liquid water up to more than 80 percent by weight. More typically, a mixture containing 50 to 80 percent by weight vapor is employed in the process of this invention. These mixtures are said to have a steam quality of 50 to 80 percent. Water from a water source such as a well, pond, stream, lake, or similar source constitutes the generator feed water. The water can be transported directly from the source into the generator or may be treated first for removal of particulate matter or dissolved scale-forming components. Since many wells are located in remote, often arid areas, a convenient source of high purity water is frequently unavailable. The steam generator may be positioned downhole, but more often is positioned at the surface of the earth in the vicinity of the injection well.
The well via which the steam is injected into the reservoir may be completed in any of a number of ways. The bottom of the well is sometimes left open with a conduit extending to near the bottom of the well, through which fluids may be injected into or produced out of the reservoir. It has, however, been discovered when steam is injected through such a system that many oil-bearing reservoirs do not have sufficient physical strength to maintain their integrity during the production phase. Such reservoirs are frequently referred to as incompetent reservoirs. Finely divided particles become dislodged from an incompetent reservoir and move with the produced fluids into the well where the particles may settle and plug the tubing or other equipment in the well. If the incompetent reservoir is under a high pressure, the high velocity of the particles as they flow with the produced fluids may cause severe erosion of well equipment.
A technique commonly employed for controlling the flow of sand from an incompetent reservoir into a well involves the forming of a gravel pack in the well adjacent to the portion of the incompetent reservoir through which fluids will pass into the well. The gravel pack is made up of coarse particles of sand, gravel, glass, cement clinker, ground nut shells, ceramic material or the like, which particles will normally range between about 21/2 and 40 mesh on the U.S. Sieve Series Scale in size. Particles between about 4 and 20 mesh are normally preferred. It is generally advantageous to employ particles falling within a narrow size range. Hence, particles of about 4 to 6 mesh, 6 to 8 mesh, 8 to 12 mesh, or 10 to 20 mesh will generally be used. The gravel pack is formed by injecting a fluid suspension of gravel or similar particles into the well to form a bed of solids containing small openings over which the reservoir sand will bridge. The flow of sands from the formation into the well during subsequent production operations is mitigated.
It is known that injection of steam into a reservoir can cause certain problems with respect to a gravel pack. M. G. Reed, "Gravel Pack and Formation Sandstone Dissolution During Steam Injection," Journal of Petroleum Technology, Vol. 32, pages 941-949 (1980). This paper describes a laboratory study of the problem that steam injected into a reservoir during oil recovery operations can dissolve large amounts of gravel pack sands and formation sandstones. In some cases, downstream reprecipitation of reaction products can damage sandstone permeability. It is suggested that this dissolution can be decreased by lowering the pH of the hot alkaline steam generator effluent or by removal of bicarbonate ions from the effluent.
While some well-treating methods have met with some success in particular applications, the need exists for a further improved steam injection treatment without substantial damage to the matrix around the well.
Therefore, it is a principal object of this invention to provide a method for enhanced oil recovery from a reservoir containing hydrocarbons, especially viscous hydrocarbons, by injecting steam into the reservoir.
It is a further object to provide such a method operable in treating wells penetrating permeable siliceous reservoirs or wells containing a gravel pack extending over the interval of the reservoir through which are passed fluids injected into or withdrawn from the reservoir.
It is a still further object to provide such a method wherein the dissolution by steam of silica from the reservoir or from the gravel pack is inhibited.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following description and appended claims.